In recent years, display technologies of various methods such as liquid crystal, plasma, organic electroluminescence and field emission have been developed in accordance with increasing demand on a thinner type TV. In any of these displays having different display methods, a transparent electrode utilizing transparent conductive film has been an indispensable constituent technology. Further, in addition to a TV, also in a touch-panel, a cell phone, an electronic paper, various types of solar batteries and various types of electroluminescence photo-modulation elements, transparent conductive film has been an indispensable technological element.
Heretofore, as transparent conductive film, various metal thin film such as Au, Ag, Pt and Cu; metal oxide thin film such as indium oxide doped with tin or zinc (ITO or IZO), zinc oxide doped with aluminum or gallium (AZG or GZO), tin oxide doped with fluorine or antimony (FTO or ATO); conductive nitride thin layer such as TiN, ZrN and HfN and conductive boronide compound thin film such as LaB6 have been known, and further, various electrodes comprising combinations thereof such as Bi2O3/Au/Bi2O3 and TiO2/Ag/TiO2 have been known. In addition to an inorganic substance, transparent conductive film utilizing conductive polymer has been also proposed (for example, refer to non-patent document 1).
However, metal thin film, nitride thin film, boronide thin film and conductive polymer film described above have been utilized only in a specific technological field such as electromagnetic wave shielding and a touch-panel field where a relatively high resistance value is allowed because characteristics of optical transparency and conductivity are not compatible.
On the other hand, metal oxide thin film is coming to be a main stream because optical transparency and conductivity can be compatible as well as durability is excellent Particularly, ITO among exemplified metal oxide compound materials is often utilized as a transparent electrode for various optoelectronics applications due to good balance of optical transparency and conductivity as well as easy formation of a micro pattern of an electrode by wet etching with an acid solution. Generally, in preparation of metal oxide thin film including ITO, a gas phase film forming method such as a vacuum evaporation method, a sputtering method and an ion plating method is utilized. However, since these film forming methods require a vacuum environment to make an apparatus big and complex as well as consume a great amount of energy for film formation, development of a technology which can reduce manufacturing cost and environmental load has been required. Further, on the other hand, a larger area of a transparent electrode film is aimed to as represented by a liquid crystal display and a touch-panel display, and accordingly, a demand to lighter weight and flexibility of a transparent electrode material has been increasing.
A method to form transparent conductive film by a liquid phase film forming method such as coating and printing by utilizing a liquid form material containing conductive micro-particles has been proposed. For example, in patent document 1, disclosed is a method to form transparent conductive film by coating a dispersion containing conductive metal oxide particles comprising such as indium oxide or tin oxide followed by being subjected to a heat treatment. Further, in patent document 2, disclosed is a film forming method in which the surface of inorganic oxide micro-particles, which have been coated on a substrate, is dissolved and followed by being stabilized by a heat treatment. Further, in patent documents 3-5, disclosed is a method to form a transparent conductive film by coating a dispersion containing such as CNT (carbon nanotube) or metal nanowire on a support.
CNT is a substance which is provided with a structure of a mono-layered or multi-layered coaxial tube form comprising a 6-member ring net work (a graphene sheet) formed by carbon, and is excellent in stability and durability. Further, the conductivity differs depending on a layer number and a structure, and mono-layered CNT exhibits the most superior conductivity. In mono-layered CNT, there are three structures depending on the difference in the orientation of a 6-member ring net work, and two of them are semi-conductive and the lest is metallic. This metallic CNT (arm-chair type CNT) is said to have conductivity comparable to copper and is preferable as a conductive material. However, since an industrial method capable of selective synthesis has not been developed and to utilize metallic CNT by selection is also practically difficult due to such as a yield of only 1% by a isolation method of metallic CNT (refer to non-patent document 2), a transparent conductive film utilizing CNT has not achieved sufficiently low resistance.
On the other hand, metal generally has a high conductivity although it differs depending on an element, and metal nanowire having a conductivity of not less than 1×107 S/m in a bulk state has been reported to be prepared by various methods such as a liquid phase method and a gas phase method. For example, referred to can be such as non-patent documents 3 and 4 as for a manufacturing method of Ag nanowire, such as patent document 6 as for a manufacturing method of Au nanowire, such as patent document 7 as for a manufacturing method of Cu nanowire, and such as patent document 8 as for a manufacturing method of Co nanowire. Particularly, since silver has the highest conductivity among metals and metal nanowire can be easily manufactured in a water phase according to non-patent documents 3 and 4, silver nanowire is regarded as the most excellent conductive material in transparent conductive film utilizing conductive fiber.
In transparent conductive film utilizing metal nanowire as an electric conductor, electric conductivity is exhibited by formation of an electric net work between metal nanowire. Since an electric conductive path of a few μm to a few tens μm long can be formed by one metal nanowire, a percolation threshold value is very small for a material containing metal nanowire to exhibit electric conductivity; therefore, compatibility of conductivity and transparency comes to be possible. With respect to conductivity, it is advantageous that metal nanowire is the longer; however, metal nanowire will be tangled to form an aggregate when it is excessively long, resulting in deterioration of transparency: Contrary, net work formation by metal nanowire becomes insufficient when it is excessively short, resulting in decrease of conductivity, and transparency will decrease when the addition amount of metal nanowire is increased to compensate conductivity. Similarly, diameter of metal nanowire also affects conductivity and transparency; and it is advantageous that the diameter is the larger with respect to conductivity, while it is disadvantageous with respect to transparency.
Therefore, for compatibility of conductivity and transparency, it is important to control the length and diameter of nanowire uniformly. However, including each non-patent document and each patent document described above, a report on a technology to uniformly control the length and diameter of metal nanowire has not been reported at all.    [Patent document 1] Japanese Patent No 3251066    [Patent document 2] JP-A 2006-245516 (hereinafter, JP-A refers to a Japanese Patent Publication Open to Public Inspection No.)    [Patent document 3] JP-A 2005-255985    [Patent document 4] Japanese Translation of PCT International Application Publication No. 2006-519712    [Patent document 5] USP 2007/0074316A1    [Patent document 6] JP-A 2006-233252    [Patent document 7] JP-A 2002-266007    [Patent document 8] JP-A 2004-149871    [Non-patent document 1] “Technologies of Transparent Conductive Film” p. 80 (Ohmsha Publishing)    [Non-patent document 2]URL:http://www.aist.go.jp/aist_j/press release/pr2006/pr20060215/pr20060215.html    [Non-patent document 3] Chem. Mater. 2002, 14, 4736-4745    [Non-patent document 4] Adv. Mater. 2002, 14, 833-837